Session D59: Magnetic Topological Insulators

Localized and collective magnetic excitations in the magnetic topological insulator Sn1-xMnxTe

The nature of the magnetism dilute magnetic topological insulators (MTIs), controlled by atomic disorder and/or clustering and the competition between short- and long-range magnetic interactions, is not well understood. MTIs that develop ferromagnetic (FM) long-range order can host dissipationless electronic transport via the quantum anomalous Hall effect. We studied the magnetic excitations in a prototypical MTI, Sn_0.95Mn_0.05Te, using inelastic neutron scattering. Neutron diffraction and magnetization data indicate that our Sn_0.95Mn_0.05Te sample has no FM long-range order. However, we observe slow, collective FM fluctuations (<70 μeV), indicating proximity to FM order. We also find a series of sharp peaks originating from excitations of antiferromagnetically coupled Mn-Mn dimers with J_AF =230 μeV. These dimers sit at next-nearest neighbor positions on the FCC sublattice, suggesting that Mn substitution is not completely random. The simultaneous presence of collective and localized components indicate that some Mn ions contribute to FM order and some form strongly-bound dimer singlets.   

Topological insulator interfaced with ferromagnetic insulators: Bi2Te3 thin films on magnetite and iron garnets

We have grown Bi₂Te₃ thin films on Y₃Fe₅O₁₂(111), Tm₃Fe₅O₁₂(111), Fe₃O₄(111), and Fe₃O₄(100) by molecular beam epitaxy with minimal chemical reaction at the interface. Electrical transport measurements were performed to study the magnetism induced by the proximity effect in the topological insulator in conjunction to the ferromagnetic insulators. We observed the anomalous Hall effect on these heterostructures. Magnetoresistance measurements at low temperature reveal a suppression of the weak antilocalization, indicating a possible topological surface state gap opening induced by the magnetic proximity effect. However, we did not observe any obvious x-ray magnetic circular dichroism (XMCD) on the Te M₄₅ edges. The results suggest that the ferromagnetism induced by the magnetic proximity effect via Van der Waals bonding in Bi₂Te₃ by is too weak to be detected by XMCD, but still can be observed by electrical transport measurements. This is consistent with published density-functional theory results on topological insulator/magnetic insulator heterostructure showing that only a small band gap of 9 meV is induced by the magnetic proximity effect.   

Tailoring Hybrid Anomalous Hall Response in Engineered Magnetic Topological Insulator Heterostructures

Magnetic topological insulators (MTIs) have greatly broadened the research scope of topological quantum materials. Introducing MTIs into the field of spintronics defines a new trend of magnetic-based logic and memory applications. Engineering the anomalous Hall effect (AHE) in emerging MTIs has great potentials for quantum information processing and spintronics applications.
In this talk, we will present the MBE growth and exotic transport properties of Bi2Te3/MnTe heterostructures. RHEED, XRR and TEM have revealed the high quality and atomically flat interface of the film. Through transport measurements, we observed pronounced AHE signals from both layers combined together The evolution of the resulting hybrid AHE intensity with the top Bi2Te3 layer thickness manifests the presence of an intrinsic ferromagnetic phase induced by the topological surface states at the heterolayer-interface. Surprisingly, we find the polarity of AHE can be switched by adjusting the Fermi level position of the Bi2Te3 layer via effective Sb-doping. Such flexibility in controlling the AHE strength, which depend on the specific band structure (i.e., Berry curvature), in our MTI heterostructures therefore opens up a multitude of opportunities to explore MTI-based spintronics devices.   

Molecular Beam Epitaxy Growth and Magnetization Characterization of Fe-doped Bi2Se3

Bi₂Se₃ is a prototypical 3D topological insulator (TI), which hosts gapless surface states protected by time-reversal-symmetry (TRS). When TRS is broken by doping with magnetic elements, this system can realize exotic electronic states, such as the quantum anomalous hall states (QAHS) (Rui Yu et al., Science 329 , 61 (2010)). We grow Fe-doped Bi₂Se₃ thin films on SrTiO₃(001) substrates by molecular beam epitaxy (MBE), and characterize them using a combination of low-temperature scanning tunneling microscopy/spectroscopy (STM/S) and magnetization measurements. We find a large difference between the Fe concentration in the topmost layer determined from STM topographs and that inferred from magnetization measurements. Moreover we find an intriguing magnetic anisotropy of the thin films, different from that observed in their bulk counterparts. Our findings provide a fresh insight into the idea of doping Fe into Bi₂Se₃ as a route towards achieving QAHS.   

Optically manipulating ferromagnetism in Cr-doped topological insulators (TIs)

Optically manipulating ferromagnetic materials has been shown to be a promising route to opto-spintronic applications. Using a combination of optically-enabled transport measurements and scanning tunneling spectroscopic (STS) measurements in the presence of circularly polarized (CP) light, we found an enhancement of magnetization in 10% Cr-doped (Bi_xSb_1-x)₂Te₃ bilayer heterostructures which consisted of a pure layer (Bi_xSb_1-x)₂Te₃ on top of a 10% Cr-doped (Bi_xSb_1-x)₂Te₃ layer. Measurements of the anomalous Hall resistance revealed an increase in R_xy and a decrease in longitudinal resistance R_xx in the bilayer magnetic TI system under CP light (wavelengths λ = 1600 ~ 1700 nm). In contrast, both R_xx and R_xy were suppressed under CP light for uniformly Cr-doped (Bi_xSb_1-x)₂Te₃. To understand the microscopic origin of these results, we performed spatially resolved STS studies on the surface state of magnetic TIs as a function of temperature and magnetic field. We further conducted optically-assisted STS studies to spatially map out the CP light-induced spectral changes to the surface state. The physical implications from correlating the spatially resolved STS under CP light with findings from macroscopic R_xx and R_xy will be discussed.   

Magneto-transport properties of bulk-insulating topological insulators (Bi,Sb)2Te3 on thulium iron garnets

Breaking time-reversal symmetry in topological insulators (TIs) via magnetic proximity effect attracted intense studies. The spin dynamics of magnetic insulators (MIs) interfacing with TIs has been investigated by ferromagnetic resonance,¹ and the magneto-transport of magnetized Bi₂Se₃ by MIs has been reported,² yet the crucial bulk-insulating TI, (Bi,Sb)₂Te₃ (BST) requires further thorough study. In this work properties of BST thin films deposited by molecular beam epitaxy on a-Al₂O₃ and tensile-strained Tm₃Fe₅O₁₂ (TmIG) of perpendicular magnetic anisotropy are reported. By adopting the low-temperature growth method,³ we demonstrated a significantly improved film growth evidenced by streaky reflection high-energy electron diffraction patterns attained at the first quintuple layer of BST on TmIG. TIs were confirmed bulk-insulating by electric transport and angle-resolved photoemission spectroscopy. We observed anomalous Hall effect up to 300K accompanied by suppressed weak anti-localization. Our work promotes the realization of quantum anomalous Hall effect at higher temperature and future TI-based devices.

1. Y. T. Fanchiang et al., Nat. Commun. 9, 223 (2018).
2. S. R. Yang et al., Phys. Rev. B 100, 045138 (2019).
3. C. C. Chen, et al., Appl. Phys. Lett. 114, 031601 (2019).   

High-temperature ferromagnetic topological crystalline insulating state induced by proximity effect in a EuS/SnTe heterostructure

Ferromagnetic topological insulators attract much attention because they enable us to realize the quantum anomalous Hall effect (QAHE), possibly useful for low-energy-consumption devices by using the chiral edge state. However, the temperature for QAHE is still low mostly due to degradation of crystallinity by doping magnetic atoms. To prevent it, using ferromagnetic proximity effect is one of the solutions [1,2]. We made and measured a EuS/SnTe heterostructure. As a result, intriguingly, the perpendicular magnetization (PM) reaches minimum at ~100 K and then increases with increasing temperature and keeps up to room temperature. A similar behavior was reported in EuS/Bi₂Se₃[1], and the explanation is that after disappearance of bulk magnetization in EuS (T_C=17 K), the direction of the interface magnetization becomes perpendicular from oblique with increasing temperature. On the other hand, when we use a trivial insulator PbTe: EuS/PbTe, the PM monotonically decreases with increasing temperature. This suggests that the anomalous interface ferromagnetism is induced by the non-trivial nature in SnTe. [1] F. Katmis et al., nature 533, 513 (2016). [2] R. Akiyama et al., arXiv 1910.10540 (2019).   

Ultrafast momentum-resolved study of electron-phonon coupling in an antiferromagnetic topological insulator

Elementary electronic and lattice excitations and their mutual interactions form the foundation of our understanding of condensed matter systems. In the context of topological insulators, the electron-phonon coupling determines in addition, the robustness of dissipationless surface states at finite temperatures. In this work, we consider the first discovered intrinsic antiferromagnetic topological insulator, MnBi₂Te₄, a system that is predicted to exhibit the quantum anomalous Hall effect [1]. We study the momentum-resolved electron-phonon coupling in this material at its inherent femtosecond timescale using ultrafast electron diffraction and coherent phonon optical spectroscopy. We find that electrons are strongly coupled to in-plane zone-boundary Eg optical phonons, resulting in a highly nonequilibrium phonon population for several hundreds of femtoseconds after excitation. The nonequilibrium phonon system subsequently relaxes by phonon-phonon coupling to zone-center transverse acoustic phonons. We simulate how the strongly coupled Eg phonons modulate the exchange interaction and magnetism using DFT calculations.

[1] J. Li, Y. Li, S. Du, Z. Wang, B.-L. Gu, S.-C. Zhang, K. He, W. Duan, and Y. Xu, Sci. Adv. 5, eaaw5685 (2019).   

Pressurizing an antiferromagnetic topological insulator candidate

Intrinsic magnetic topological insulators hold the potential of hosting quantum anomalous Hall states, chiral Majorana fermions, and topological magnetoelectric effects. We have identified Zintl antiferromagnet Eu₅In₂Sb₆ as a candidate material which displays remarkable insulating behavior. Our previous results showed the presence of colossal magnetoresistance in this system, which is consistent with the presence of magnetic polarons. Here we investigate the effects of applied hydrostatic and uniaxial pressure on Eu₅In₂Sb₆ single crystals in an effort to drive the system through a topological phase transition. Our results show an exceptionally large decrease in resistivity under applied hydrostatic pressure as well as uniaxial pressure along the c axis.   

Spin-to-charge conversion on the edge of quantum spin Hall insulator

We present our theoretical work on dynamical spin-to-charge conversion at the edge of a quantum spin Hall insulator (QSHI), namely a two-dimensional topological insulator with helical edge states. Interconversion between spin- and charge-related quantities has been a key idea in making use of magnetic materials, especially in the context of spintronics. QSHI is a typical system showing a universal charge-to-spin conversion behavior, namely the quantum spin Hall effect, whereas the spin-to-charge conversion therein is still not clearly understood.

We here investigate the many-body dynamics of the electrons under the magnetization dynamics at the QSHI-ferromagnet(FM) junction. We analytically treat the electron dynamics in terms of the Floquet-Keldysh formalism, and compare the spin injection rate from the FM into the QSHI and the charge current induced along the edge. Whereas the edge current seen in the previous works is reproduced, we find that it is not proportional to the spin injection rate, especially when the exchange interaction at the junction is strong enough. This relation implies that the spin-to-charge conversion in this system cannot be considered as the inverse spin Hall effect, while it can be rather seen as the inverse Edelstein effect.   

Exchange interactions between topological and meta-magnetic insulators

The hallmark of 3D topological insulators(TIs)are topological surface states(TSS)protected by the time-reversal symmetry.One of the important goals is the ability to manipulate surface electron states and to lift the topological protection of TSS for the emergence of novel physical phenomena.Gapping of TSS has been demonstrated in TIs with bulk doping using the magnetic impurities and via exchange coupling to insulating magnetic or antiferromagnetic materials.Here we report an experimental study of exchange coupling in EuSe/Bi₂Se₃ and EuSe/Bi₂Se₃/EuSe heterostructures grown by MBE.Bi₂Se₃ is a typical TI and EuSe is a meta-magnetic insulator with a rich phase diagram of paramagnetic,antiferromagnetic,ferrimagnetic and ferromagnetic phases at low temperatures.Polarized neutron scattering experiments indicate enhanced exchange interaction between EuSe and Bi₂Se₃ and we observe finite in-plane magnetization at the interface even above the Néel temperature (T_N~5K) of EuSe.In transport measurement,we observe modulation of longitudinal and Hall resistance as a function of the magnetic field,which may be explained by the formation and re-arrangement of magnetic domains at the interface.   

Characterization of the topological magneto-electric effect in Bi2Se3 topological insulator

We investigate the topological magneto-electric effect (TME) in Bi₂Se₃ topological insulator (TI) thin films, with a perpendicular exchange field applied to the top and bottom surfaces in the anti-parallel configuration. Here the system is in the axion insulator phase, which allows a direct probe of the quantized topological θ-term of the axion electrodynamics. The θ-term is a TI bulk property which gives the coefficient of proportionality between an applied electric field and the induced magnetization. We first consider an infinite slab and extract θ in a tight-binding framework, under the condition of weak electric field. In the second part of the work, we consider systems which are finite in one or more dimensions. By analyzing the dependence of θ on the system size and the strengths and profiles of the exchange fields on the two surfaces, we determine when the TME is expected to be quantized. Furthermore, we investigate possible deviations from exact quantization in the regime where the exchange field on one of two surfaces is progressively enhanced beyond a critical value. Our study sheds light on the fundamental issue of whether TME in a TI is completely determined by the bulk properties or it can be affected by how time-reversal symmetry is broken at the surfaces.   

Electrically Tunable Anomalous Hall Effect in Topological Crystalline Insulator Films

Introducing ferromagnetism into topological crystalline insulator SnTe could lead to the high-Chern-number quantum anomalous Hall effect. Here we report the observation of magnetic proximity effect in heterostructures formed by the SnTe (111) and 2 quintuple layers thick Cr-doped (BiSb)₂Te₃ (CBST). Owing to the charge transfer between CBST and SnTe, the Fermi level of the latter can be tuned by adjusting the Bi/Sb ratio in the former. An anomalous Hall resistance as large as 0.08 h/e² is revealed. The transferred charges modify the electric field perpendicular to the film and change the critical thickness of topological phase transition in SnTe. Such property makes SnTe useful in realizing the topological transistor.
  

Large-Gap Quantum Anomalous Hall effect in a Magnetically doped Type-I Topological Heterostructure

Current method of doping a topological insulator (TI) with magnetic element has only yielded low temperature quantum anomalous hall effect due to the small exchange energy of dopants and uncontrollable shifts in the Fermi-level due to band-bending effects [1]. To overcome the band-bending effect, Fermi -level can be pinned by creating a Type-I type TI/insulator band offset, while the magnetic exchange energy can be increased by searching over different potential magnetic dopants that remain neutral. Given the recent successes of growing high-quality TI-films on Cr₂O₃ heterostructure [2], we study different magnetically doped TI/Cr₂O₃ heterostructures using a combination of density functional theory and k.p modelling. We find Sb₂Te₃ to form a type-I interface with Cr₂O₃l, thereby making the heterostructure insulating, meanwhile maximizing the Zeeman energy due to the segeration of the dopants at the interface, suggesting possible room-temperature QAHE. We also compare our interfacial magnetism model with available experiments.

[1] X. Feng et al, Adv. Mater. 28, 6386 (2016).
[2] F. Wang et al, Nano Lett. 19, 2945 (2019).